It has long been recognized that cardiac defibrillation—the termination of an episode of fibrillation—can be accomplished through application of an electrical shock to the cardiac muscle and that fibrillation can be induced by stimulating the ventricles during the vulnerable zone. See, for example, Swartz et al., “Influence of T-Wave Shock Energy on Ventricular Fibrillation Vulnerability in Humans,” Journal of American College of Cardiology, 1995 Conference Abstracts, February 1995; see also, Karolyi et al., “Timing of the T-Wave Shock for Inducing Ventricular Fibrillation in Patients With Implantable Cardioverter Defibrillators,” PACE NASPE Abstracts, Vol. 18, April 1995 (Part II), p. 802. Numerous types of defibrillating devices, both external and implantable, are available for the purpose of cardiac defibrillation through electrical stimulation.
When implanting an implantable medical devices for defibrillating a patient's heart, such as an implantable defibrillator device or an implantable pacemaker/cardioverter/defibrillator device, for example, it is desirable to test the device's operability to ensure that it is capable of reliably defibrillating the heart. One method of testing a defibrillator's operability to ensure that it is capable of reliably defibrillating the heart involves first inducing an episode of fibrillation in the patient's heart, and then activating the defibrillator to ascertain whether it is capable of terminating the induced fibrillation. Current implantable defibrillators operate to induce fibrillation in either chamber of the heart (atrial or ventricular) by delivering a stimulus during that chamber's repolarization phase, i.e., within a so-called “vulnerability window” following the chamber's depolarization period when the heart has begun to repolarize but has not completely repolarized. This is described, for example, in U.S. Pat. No. 5,129,392 to Bardy et al., entitled “Apparatus for Automatically Inducing Fibrillation,” which patent is assigned to assignee of the present invention and hereby incorporated by reference herein in its entirety.
In order to maximize efficiency of the implantable medical device, it is necessary to determine as accurately as possible the minimal energy level necessary to defibrillate the patient's heart, i.e., the defibrillation threshold (DFT) after the device and leads are implanted. The upper limit of vulnerability (ULV) has been used as an alternate to defibrillation threshold testing. The upper limit of vulnerability is defined as the minimum strength of a shock delivered into the vulnerable zone of the cardiac cycle that will not result in the induction of ventricular fibrillation. Timing the shock so that it occurs in the vulnerable zone is critical since any shock strength delivered outside the vulnerable zone will not fibrillate, resulting in an underestimate of the defibrillation threshold. As a result, one of the difficulties involved in employing an electrical shock to the cardiac muscle to induce fibrillation is determining the timing interval best suited for delivering the shock. Current methods for determining the timing or coupling interval of the shock involve identifying a point on the patient's T-wave and delivering shocks with respect to that point. Often, several coupling intervals are scanned in the vicinity of the T-wave to be sure the vulnerability zone has not been missed. Extra shocks mean extra time and possible extra discomfort to the patient during implant testing of the implantable medical device.